In an attempt to keep the hogs in view longer, I made a pig pipe. I took a wild guess on proper hole size and did 5/8”. Looked about right, but I just guessed.

Pt corn in it and see how it flows. You want the corn to just be able to come out without letting too much out. I think, and I stress think, I wound up with 5/8". I started small and gradually increased the size to get the flow I wanted. Dang, that almost sounds dirty. I think you'll be fine. If it's letting too much out just put some duct tape over a few of the holes to slow the flow.

I make them 1/2".

I second the 1/2". I did mine at 5/8 to start which was a bit too large. Then the field mice, etc., decided that wasn't large enough for them and chewed them out even larger. Way too much corn flows out now. Thinking about using some 5 minute epoxy to stick some 1/2" washers over those.

I built an 8 ft long one about 20 or so years ago. Staked it down with chain so it could only go in circles. I kept it about 10 years until a thief came along who needed it more than me. I got him on game cam but he was masked.

Use heavy duty PVC because squirrels and other varmints will gnaw around the holes. Pigs can empty them because they just keep it rolling. I like the idea of putting one near a feeder. That way, when it gets emptied, wildlife will keep coming to it for corn.

I think I started with the 5/8 and it has worked well. I also have had the mice/squirrels enlarge them. I've had mine out for about 13-14 years and never replaced the pipes. I have had to replace one of the eye bolts and cables before. They go through some abuse by the hogs. Money well spent. I used one reg length stick of schedule 40 and cut in have to make two.

What size pipe would be the best? I'm thinking at least 4 inch inside.

If you want them in view longer, corn hole them

Dig a few holes with post hole diggers 3 or so feet deep, fill up with corn, add anything else you want.
Won't have as much of an issue with coons, mice, squirrels, etc.